704 



ECOLOGY AND EVOLUTION 



to alternate hosts show how evolution 

 brings about adjustment to the community 

 as a system, and also indicate the long 

 duration of the associations and relations 

 within the community. Of course, these 

 organisms are not only adjusted to different 



In summary, we may say that reciprocal 

 evolution of exploited and exploiting forms 

 has occurred, that the selection pressures 

 through exploitation gradually sort organ- 

 isms in relation to each other, and that 

 these evolutionary relationships create 



Fig. 251. Life cycle of a nematode {Cammallanus trispinosis). The larval worms (1) pass 

 out of the intestine of a turtle with the feces and are eaten by Cyclops. The worms ( 2 ) develop 

 in the body cavity. If the Cyclops is eaten by a damsel fly, dragon fly, fish or newt, the 

 worms (3) attach to the intestinal walls, and if these hosts are eaten by a turtle, the adult 

 worms (4) infect the small intestines. (From Thou ins.) 



biotic aspects of the community, but show 

 many adaptations to the physical factors as 

 well. In other words, a combination of fac- 

 tors in the ecosystem has exerted selection 

 pressures guiding the evolution of organ- 

 isms toward adaptation to the system as a 

 unit. 



highly important interacting, interdepend- 

 ent, integrated systems of species. 



TOLERATION 



It has already been mentioned (p. 699) 

 that extreme disoperation tends to be eUm- 

 inated through natural selection, A predator 



